Often used to describe a friend who has either made a silly mistake or recently done you a favor.
It is also important to note that when calling someone a Kunt Muffin it is usually a good idea to specify that you are using the complimentary form of the phrase to avoid confusion.
Sam: Aw, that is so sweet!
Carly: She is such a kunt muffin!
Sam: That isn't very nice!
Carly: No, no I mean a kunt muffin with a K.